The present invention relates to the structure of a semiconductor chip which comprises a semiconductor substrate and a metal radiator provided on the rear surface of the semiconductor substrate.
As illustrated in FIG. 1, a metal radiator plate 4 is bonded to the rear surface of the semiconductor substrate of a semiconductor chip which generates a large amount of heat. In FIG. 1, numeral 1 designates a semiconductor chip, numeral 2 designates a semiconductor substrate, numeral 3 designates an active region, and numeral 4 designates a radiator plate. Such a semiconductor chip 1 is housed within a package (not shown) so that it constitutes a semiconductor device.
The radiator plate 4 is bonded to the semiconductor substrate 2 at a high temperature. When the semiconductor substrate 2 is restored to room temperature after bonding, as illustrated in FIG. 2A, the radiator plate and the semiconductor substrate 2 are convexly curved or twisted due to the difference between the coefficient of heat expansion of the radiator plate 4 and that of the semiconductor substrate 2. The semiconductor chip 1 is made of silicon (Si) or gallium-arsenic (GaAs). The coefficient (.alpha.) of heat expansion of silicon is 2.4.times.10.sup.-6 (in cm/oc, throughout). The coefficient (.alpha.) of heat expansion of gallium-arsenic is 6.9.times.10.sup.-6. The radiator plate 4 is made of copper (Cu) or silver (Ag). The coefficient (.alpha.) of heat expansion of copper is approximately 16.7.about.20.times.10.sup.-6. The coefficient (.alpha.) of heat expansion of silver is about 19.about.20.5.times.10.sup.-6. Such deformation of the semiconductor chip 1 causes the semiconductor chip 1 to have unstable characteristic and causes the semiconductor substrate thereof to crack. Furthermore, the semiconductor chip 1 cannot be reliably handled with a vacuum chuck since the surface of the semiconductor substrate 2 is also curved.
Also, as illustrated in FIG. 2B, when a heat treatment is carried out, to bond the semiconductor chip 1 to the package 5, the semiconductor chip is concavely curved due to the difference between the coefficient of heat expansion of the semiconductor substrate 2 and that of the radiator plate 4. This hinders a tight contact between the semiconductor chip 1 and the package 5 so that the efficiency of heat radiation is degraded. Numeral 6 designates solder for securing the chip 1 to the package 5.
The dimension of a semiconductor chip 1 having a high frequency or high output power is large. The thickness of the semiconductor substrate 2 of such a semiconductor chip 1 should be large from the viewpoint of strength and easy handling of the semiconductor chip 1. However, a thick semiconductor substrate 2 causes a degradation in the efficiency of heat radiation. A thin semiconductor substrate 2 is desirable for increasing the output power of the semiconductor chip 1 since generated heat is effectively radiated. However, the aforementioned problem of deformation of the semiconductor chip 1 is accentuated if the semiconductor substrate is thin.